The present invention resides in a container for storing a hydrated phase change material (PCM). More particularly, the invention resides in a container of a synthetic resinous material which is made by rotomolding to form a highly stress resistant container for the hermetic storage of hydrated PCM's .
In the past, various types of thermal energy storage materials or phase change materials have been used in a variety of heating or cooling installations. Such uses have been, for example, in thermal energy storage applications such as water heating, solar heating, storage of coolness during off-peak energy use and release of coolness during periods of peak power use, and the like.
PCM's are greatly preferred as a thermal energy storage material since they will absorb large amounts of energy with no change in temperature in their melting phase change. Accordingly, the use of a PCM allows for a much greater energy storage per unit volume compared to sensible heat storage mediums such as water or rocks. For example, sodium sulfate decahydrate (Glauber's salt) is a well known PCM which absorbs a large amount of energy before it melts at a temperature of 90.degree. F. In the temperature range of from 85.degree. to 110.degree. F., the PCM will store about 5 times more energy than water and 17 times more than rock for an equal volume of material. Accordingly, the storage volume with a PCM is greatly reduced while the thermal energy storage efficiency is substantially improved. Any number of well known hydrated PCM's that melt and freeze over a desirable temperature range and which are readily available, may be used in the container of the invention. Typical PCM's are those listed, for example, in ASHRAE Journal of September, 1974, in an article entitled "Solar Energy Storage" by M. Telkes.
The storage of PCM's has been extensively investigated in past years since containers for PCM's must be able to withstand various stresses over a long period of time during which the PCM undergoes innumerable freeze-thaw cycles. Note Report ORO/5217-8 of November, 1978 entitled "Macro Incapsulation of Phase Change Materials," authored by G. A. Lane et al. The study was conducted under the auspices of DOE. Studies generally have shown that containers for hydrated PCM's must be constructed of a durable material and must be reliably leak-proof to liquids and vapors. Since PCM's are generally corrosive, the containers must also be constructed of a material which is not corroded by a particular PCM.
Metallic containers or cans made of coated steel or aluminum have been reliably used for foods or beverages. Usually, the containers have at least one crimped end closure. Such containers are not suitable, however, for use in the storage of PCM's since cycling tests have shown that repeated melting and solidification of the PCM, gradually caused leakage through the sealed rims. Similar results were observed with soldered seams which proved to be unsatisfactory as well. Dissimilar metals produced an electric potential during contact with the PCM thereby producing a "battery effect" that resulted in the corrosion of the metal container. Other materials such as stainless steel or corrosion resistant metals may prove to be effective over long periods of time but their cost is prohibitive and thus are an impractical alternative to other low cost materials.
An attractive alternative to metal containers have been containers made from synthetic resinous materials. A container in the form of a sealed cylinder of high density polyethylene is disclosed in U.S. Pat. No. 4,299,274 (S. Campbell), issued Nov. 10, 1981. However, such elongated tube-like containers are separately provided with fusion welded caps to seal the open ends of the tubular storage container. Alternatively, the open ends are heated and pinch sealed under fusion. Angled or non-linear pinch configurations are described to minimize any tendency for thermal distortion. Nevertheless, in all of the described sealing methods, the seals are vulnerable and have a tendency to crack under the continued flexural stress to the wall of the tube during freeze-thaw cycling of the PCM within the tube. Moreover, the tube-like containers described in the patent to Campbell have a relatively low surface area to volume ratio and consequently do not collect or distribute heat as well as trays or panels of a generally flat and rectangular shape which allow for an improved surface to volume ratio for storing and releasing thermal energy.
Various other types of systems for containing PCM's are described in, e.g., U.S. Pat. No. 2,595,905 (M. Telkes), issued May 6, 1952; U.S. Pat. No. 3,720,198 (N. Laing et al.), issued Mar. 13, 1973; U.S. Pat. No. 4,237,023 (T. E. Johnson et al.), issued Dec. 2, 1980; U.S. Pat. No. 4,259,401 (D. Chahroudi et al.), issued Mar. 31, 1981; U.S. Pat. No. 4,277,357 (B. J. Boardman), issued July 7, 1981; U.S. Pat. No. 4,290,416 (T. Maloney), issued Sept. 2, 1981; and U.S. Pat. No. 4,337,754 (S. J. Conger), issued July 6, 1982.
In an article entitled "Heat of Fusion Systems for Solar Heating and Cooling"; Solar Engineering of September, 1977, pages 26-29, the author, M. Telkes, describes various containers that may be used for the storage of PCM's. With respect to plastic containers, the author included thermoformed or blowmolded containers which may be tray-like units and in which the trays "must be used horizontally and can be stacked with spacers". Such plastic containers are said to be formed of high density polyethylene or polypropylene combined with certain additives. Trays of the type referred to hereinabove were manufactured by Solar, Inc. of Mead, Neb. and are described in Solar Engineering of April, 1980, p. 44.
It has been found, however, that containers for PCM's still are not entirely satisfactory if manufactured by the standard methods of molding.